The continuing increase in the use of coated components is due to their improved performances also in presence of a high variety of substrates on which the coating can be deposed. In fact the durability of coated components is usually higher than uncoated ones especially when wear and corrosion resistance must be improved [14-16]. Among the coating technologies a well known position is assured from the PVD process used not only on several substrates (both ferrous and non ferrous alloys can be coated with PVD layers) but also in different chemical compositions (from the traditional TiN coatings to the more complex ceramic multilayer tailored for peculiar applications). Metallurgical studies on PVD coatings have been focused on the increasing of the mechanical properties such as fatigue, thermal fatigue, contact fatigue and toughness by controlling chemical composition, hardness and internal stress distribution ceramic layers. Nowadays the mechanical behaviour of newly developed PVD-coated components can be achieved by means of the so called "duplex treatment" [1-5] consisting in a thin PVD film deposed on a nitrided steel substrate. The nitrided case minimises the substrate deformation under the service applied loads while the high hardness and low friction coefficient characterizing the PVD coating increase the wear performance of the substrate treated parts. In the present research work a 42CrMo4 steel was considered in the following conditions: A) quenched and tempered, B) quenched and tempered + CrN PVD coating, C) nitrided D) nitrided + CrN PVD coating (duplex treatment). The functional coating was obtained by means of an arc device settled to obtain a 5 um total thickness. In order to verify the coating properties under dynamic loading and to simulate a wide range of tribological damage conditions, an "Impact Test" apparatus was appositely arranged at the Politecnico of Milan (Fig. 1). During the Impact test each specimen was cyclically loaded up to 5×104 cycles by a spherical indenter (diameter 6 mm) [18-22]. At the end of each test samples were observed by means of a scanning electron microscope (SEM) in order to investigate the mechanism of coating failure that resulted adhesive or cohesive. The mechanical characterization included also hardness and microhardness investigations carried out in order to measure the hardness profiles and the Young's modulus of all considered samples. Using a rotating bending machine, fatigue tests were also carried out in the different conditions induced by thermochemical treatments and /or coatings. Experiments were executed at room temperature, in air, at a test frequency of 33 Hz using a sinusoidal load waveform and a load ratio R=0. The endurance limit was set after 3×106 stress cycles. Results of the fatigue tests were analyzed according to the stair-case up and down method. The fracture surfaces of the cycled samples were also examined by SEM in order to identify the microstructural parameters governing the fatigue crack nucleation A good adhesion of the CrN layer was detected (Fig. 3) for all the examined substrate conditions; in addition no significant modification of the substrate mechanical properties can be reported (microhardness data on Fig. 2). The impact test shows a better resistance for the duplex treated specimens with respect both nitrided samples and quenched and tempered + PVD coated ones. Impact craters obtained at low loading applied stress (500 N and 5×104 cycles) do not show any cracks (Fig. 6-7); on the contrary when the maximum load is applied (1000N), several cracks affect the duplex treated surface. The stress gradient value calculated from the difference between the coating Young's modulus and the substrate Young's modulus can be used to interpret the experimental results: the condition minimizing this difference minimizes the impact damage too (Table 3). In all the considered conditions the rotating bending fatigue test (Table 2) allows the PVD tested to increase the fatigue limit even if in different percentages with respect to uncoated specimens. In particular, considering the nitrided 42CrMo4 steel grade, good fatigue behaviour was detected without any additional coating step. On the contrary a beneficial effect of PVD coating is evident in the quenched and tempered condition where the surface residual stress gradient due to the heat treatment is not able to produce a high level of compression stress and therefore to increase the fatigue behaviour. In conclusion the introduction of a duplex treatment instead of a less expensive nitrided treatment has to be certainly considered for its positive effect in terms of improved wear resistance and load capability while can not be proposed only on the basis of a significant increases in the fatigue behaviour.
Impact test and fatigue behavior of duplex and non duplex tretments
Lecis N.;La Vecchia G. M.
2007-01-01
Abstract
The continuing increase in the use of coated components is due to their improved performances also in presence of a high variety of substrates on which the coating can be deposed. In fact the durability of coated components is usually higher than uncoated ones especially when wear and corrosion resistance must be improved [14-16]. Among the coating technologies a well known position is assured from the PVD process used not only on several substrates (both ferrous and non ferrous alloys can be coated with PVD layers) but also in different chemical compositions (from the traditional TiN coatings to the more complex ceramic multilayer tailored for peculiar applications). Metallurgical studies on PVD coatings have been focused on the increasing of the mechanical properties such as fatigue, thermal fatigue, contact fatigue and toughness by controlling chemical composition, hardness and internal stress distribution ceramic layers. Nowadays the mechanical behaviour of newly developed PVD-coated components can be achieved by means of the so called "duplex treatment" [1-5] consisting in a thin PVD film deposed on a nitrided steel substrate. The nitrided case minimises the substrate deformation under the service applied loads while the high hardness and low friction coefficient characterizing the PVD coating increase the wear performance of the substrate treated parts. In the present research work a 42CrMo4 steel was considered in the following conditions: A) quenched and tempered, B) quenched and tempered + CrN PVD coating, C) nitrided D) nitrided + CrN PVD coating (duplex treatment). The functional coating was obtained by means of an arc device settled to obtain a 5 um total thickness. In order to verify the coating properties under dynamic loading and to simulate a wide range of tribological damage conditions, an "Impact Test" apparatus was appositely arranged at the Politecnico of Milan (Fig. 1). During the Impact test each specimen was cyclically loaded up to 5×104 cycles by a spherical indenter (diameter 6 mm) [18-22]. At the end of each test samples were observed by means of a scanning electron microscope (SEM) in order to investigate the mechanism of coating failure that resulted adhesive or cohesive. The mechanical characterization included also hardness and microhardness investigations carried out in order to measure the hardness profiles and the Young's modulus of all considered samples. Using a rotating bending machine, fatigue tests were also carried out in the different conditions induced by thermochemical treatments and /or coatings. Experiments were executed at room temperature, in air, at a test frequency of 33 Hz using a sinusoidal load waveform and a load ratio R=0. The endurance limit was set after 3×106 stress cycles. Results of the fatigue tests were analyzed according to the stair-case up and down method. The fracture surfaces of the cycled samples were also examined by SEM in order to identify the microstructural parameters governing the fatigue crack nucleation A good adhesion of the CrN layer was detected (Fig. 3) for all the examined substrate conditions; in addition no significant modification of the substrate mechanical properties can be reported (microhardness data on Fig. 2). The impact test shows a better resistance for the duplex treated specimens with respect both nitrided samples and quenched and tempered + PVD coated ones. Impact craters obtained at low loading applied stress (500 N and 5×104 cycles) do not show any cracks (Fig. 6-7); on the contrary when the maximum load is applied (1000N), several cracks affect the duplex treated surface. The stress gradient value calculated from the difference between the coating Young's modulus and the substrate Young's modulus can be used to interpret the experimental results: the condition minimizing this difference minimizes the impact damage too (Table 3). In all the considered conditions the rotating bending fatigue test (Table 2) allows the PVD tested to increase the fatigue limit even if in different percentages with respect to uncoated specimens. In particular, considering the nitrided 42CrMo4 steel grade, good fatigue behaviour was detected without any additional coating step. On the contrary a beneficial effect of PVD coating is evident in the quenched and tempered condition where the surface residual stress gradient due to the heat treatment is not able to produce a high level of compression stress and therefore to increase the fatigue behaviour. In conclusion the introduction of a duplex treatment instead of a less expensive nitrided treatment has to be certainly considered for its positive effect in terms of improved wear resistance and load capability while can not be proposed only on the basis of a significant increases in the fatigue behaviour.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
